The present invention relates to a specimen stage apparatus suitable as a specimen stage of an electron microscope used especially for inspecting and evaluating semiconductors in the field of semiconductor manufacture, an electron microscope provided with the specimen stage apparatus and a specimen stage apparatus positioning control method.
With semiconductor devices minified in recent years, not only the manufacturing apparatus but also the inspecting/evaluating apparatus has been required of high accuracy comparable to that of the manufacturing apparatus. Typically, in order to evaluate whether or not the shape critical dimension of a pattern formed on a semiconductor wafer is correct, a scanning electron microscope with the critical dimensioning function (hereinafter referred to as a critical dimensioning SEM) has been used.
In the critical dimensioning SEM, an electron beam is irradiated on a wafer, a captured secondary electron signal is processed to an image and the edge of a pattern is determined from a bright to dark change or vice versa to extract a critical dimension. In this case, it is important for coping with the minified formation of the semiconductor device to obtain a secondary electron image of less noise at high magnifications for observation. Accordingly, many secondary electron images need to be superimposed on one another to improve the contrast and to this end, the specimen stage carrying and holding the wafer is required to be suppressed in its vibration and drift in the order of nanometer.
Achieving the aforementioned suppression in vibration and drift necessitates a specimen stage apparatus capable of performing highly accurate positioning and generally, this type of specimen stage apparatus is comprised of a servo-control system adopting a linear motor as drive mechanism.
In carrying out the servo-control, however, a slight or minute vibration is occurring in the order of nanometer even in the stop condition, so that the specimen stage vibrates to vary the specimen position and besides, the vibration of the specimen stage causes the whole apparatus to vibrate and therefore, the secondary electron image will sometimes be affected adversely.
As a countermeasure against a minute vibration attributable to the servo-control as above, a technique is available in which a braking structure is provided for a movable table of the specimen stage and when stopping the specimen stage, braking is activated to turn off the servo-control (see JP-A-2001-88695 (paragraphs [0010] to [0016], FIGS. 1 to 4), for example). In the known technique, the movable table is provided with a brake driven by an air cylinder and the movable table clamps together, with the help of braking, a plate member laid along the direction in which the movable table moves, thus making it possible to brake the movable table.
In another technique different from the above one, a movable table is provided with a rolling member acting on the base surface and when stopping the specimen stage, the thrust force of the rolling member applied on the base surface is increased with the help of a force generated by a piezoelectric device provided for the movable table, so that the movable table is braked (see JP-A-8-222500 (paragraphs [0025] to [0028], FIGS. 4 to 5), for example).